It is known to use a pigment having been subjected to a surface treatment for imparting water dispersibility (hereinafter referred to as a water-dispersible surface treated pigment) as a colorant of a water-based pigment ink composition for, for example, inkjet recording. JP-A-8-3498 (The term “JP-A” used herein means an “unexamined published Japanese patent application”.) discloses a technique of introducing a carboxylic acid salt to the surface of carbon black particles to improve dispersion stability. JP-W-A-2000-512670 (The term “JP-W-A” used herein means an “international patent application published in its Japanese national stage”) discloses an inorganic or organic pigment having an ionic group (or an ionizing group) introduced to its surface, wherein the ionic group includes a carboxyl group, a sulfonic acid group, and their salts.
However, an ink composition containing such a water-dispersible surface treated pigment as a colorant has a disadvantage that color developability can not be improved further than a certain level for the following reason. Because the water-dispersible surface treated pigment has insufficient fixability onto a recording medium, a resin emulsion should be used in combination. The fixability insufficiency problem of the water-dispersible surface treated pigment is particularly noticeable on paper designed for inkjet recording (inkjet paper). In an attempt to improve color developability of an ink composition comprising such a water-dispersible surface treated pigment and a resin emulsion, the pigment content could be increased, which necessitates increasing the resin emulsion content, too. Since the increases in these solid contents result in an increase in viscosity of the ink composition, there is an upper limit in the pigment content. Therefore, the pigment content cannot be increased enough to reach satisfactory color developability.